1. Field of the Invention
The present invention relates to packaging assemblies for semiconductor integrated circuits. The present invention relates generally to electronic devices and assemblies. More particularly, the present invention relates to methods and structures for attaching a heatspreader in high thermal performance integrated circuit (IC) packages. Even more particularly, the present invention includes methods and structures for adhesively attaching high coefficient of thermal expansion (CTE) heatspreaders directly to the back of a die while maximizing thermal performance, mechanical integrity and reliability of the assembly.
2. Description of Related Art
Heat produced by integrated circuits through power dissipation must be conducted away from the integrated circuit. Otherwise, the circuit will be subjected to excessive thermal excursion and most likely will be damaged. One approach for conducting heat away from an integrated circuit is to attach the integrated circuit to a heatspreader which transmits the heat to a large surface cooled by air.
In the past, heatspreaders have been made of expensive materials and have had to be specially shaped and/or attached to the IC package using complicated and expensive assembly techniques. One example of such special shaping involves several specially shaped heatspreaders. These non-planar configurations are expensive to manufacture and require precise mounting to operate correctly and to minimize damage to the semiconductor device to which they are attached. Moreover, the heatspreader is in essentially direct contact with the die, again complicating the fabrication of the device and increasing the cost of manufacture and the likelihood of damage to the semiconductor device.
Additionally, heatspreaders have been made of expensive alloys and other materials, typically having low CTEs (in the range of 2.0 to 6.5 ppm/C) to minimize thermomechanical stresses . The CTE mismatch between the heatspreader and the die is relatively small, not more than 4.0 ppm/C. As a result, the thermochemical stresses induced during typical reliability tests such as temperature cycling (TMCL) and autoclave/pressure chamber testing (ACLV/PCT) are relatively small and do not pose significant reliability problems. A major disadvantage of these materials is that special elements need to be alloyed into the heatspreaders in order to keep the CTE low, and this typically makes the heatspreaders quite expensive.
High CTE heatspreaders such as aluminum and copper are typically inexpensive compared to the low CTE materials mentioned above. These high CTE materials typically are more than 6 times less expensive than the low CTE materials. However, high CTE materials pose problems when they are attached directly to low CTE materials like the silicon die. The CTE mismatch between the heatspreader material and the silicon die causes delamination at the interfaces of the attachment material due to the thermomechanical stresses, thus adversely affecting the overall thermal performance as well as the mechanical integrity of the package. The invention disclosed and claimed herein includes techniques to overcome these drawbacks and successfully achieve a mechanically durable and reliable assembly with excellent thermal performance for high CTE heatspreaders. This enables the user to successfully take advantage of the low cost offered by high CTE heatspreaders.
The present invention includes methods and structures for adhesively attaching high coefficient of thermal expansion (CTE) heatspreaders directly to the back of a die while maximizing thermal performance, mechanical integrity and reliability of the assembly, thereby overcoming the shortcomings of earlier methods and structures.
The present invention uses improved structures and methods for assembling heatspreader attachments in integrated circuit packages, permitting attachment of a relatively low cost heatspreader having a high coefficient of thermal expansion directly to the back of a die while maximizing thermal performance, mechanical integrity and reliability of the assembly. The improvements are realized through the use of heatspreader geometries, adhesive geometries, assembly techniques and underfill geometries and specific adhesive materials to attach the heatspreader to the die.